Performance mainly required of electrically insulating oils such as capacitor oils include, the dielectric breakdown voltage being high, the hydrogen gas absorbency being high, the viscosity being low, and the melting point being low, and in addition the compatibility to various types of members such as packings and dielectric members constituting capacitor elements being good. In recent years, as various types of oil-filled electrical equipment (oil-impregnated electrical equipment), including capacitors, has been used all over the world, electrically insulating oils having higher dielectric breakdown voltage have been being used. With the development of the world economy, use in extremely low temperature areas where oil-impregnated electrical equipment has not been used so far is required of the oil-impregnated electrical equipment, and studies on electrically insulating oils excellent in low temperature characteristics that can adapt to the use are promoted. On the other hand, in the process of conventional capacitor performance improvement, a polypropylene film having a high dielectric constant is used as the dielectric between the electrodes instead of paper. In the insulating oil having high electrically insulating properties (high dielectric breakdown voltage) as described above, compounds constituting it are generally aromatic compounds, and therefore the insulating oil is a factor that swells the polypropylene film, and decreases the dielectric breakdown voltage of the capacitor due to the extension of defects in the film, an increase in the mechanical stress of the film in the capacitor strongly wound, and the like. This influence is significant particularly at high temperature, and in industrial equipment installed outdoors, electrical characteristics at high temperature are also important because of its use form.
As the electrically insulating oils having high dielectric breakdown voltage, mixtures of benzyltoluene and dibenzyltoluene have been used over a long time. Benzyltoluene has a high ratio of aromatic carbon in the molecule, has high hydrogen gas absorbency, and is excellent in withstand voltage characteristics, and therefore is widely used in electrically insulating oils.
In addition, in the following Patent Literature 1, electrically insulating oils composed of benzyltoluene and ditolylmethane are described, and it is found that the performance changes greatly depending on the blended substances and their blending proportion. In other words, there are rare cases where in electrically insulating oils, such performance as not considered in theory is obtained depending on the blended substances.
On the other hand, for 1-phenyl-1-xylylethane or 1-phenyl-1-ethylphenylethane, its production is easy, and it has excellent characteristics such as the dielectric breakdown voltage being relatively high, and the dielectric loss being small, and therefore it is widely used in electrically insulating oils. For example, as an electrically insulating oil composition for which in addition to the dielectric breakdown voltage being excellent and the dielectric loss being small, the oxidation stability is particularly excellent, a composition containing 1-phenyl-1-(2,4-dimethylphenyl)ethane or 1-phenyl-1-(2,5-dimethylphenyl)ethane is proposed (see the following Patent Literature 2).